1. Field of the Invention
The present invention relates to an operation apparatus for correcting ametropia of the eye by correcting the shape of a cornea with laser beam, and particularly to an operation apparatus for correcting hypermetropia by reducing the curvature of the cornea and increasing its refracting power.
2. Description of Related Art
Recently known is a method of correcting ametropia of the eye by eliminating a part from the surface of a cornea by laser beam, and thereby changing the curvature; called Photo-refractive Keratectomy. The method is currently utilized for only correction of myopia, hardly utilized for correction of hypermetropia. The reason is considered as follows.
In the case of correcting myopia, as shown in FIG. 1, the surface of the cornea E.sub.c is eliminated by laser beam so that the thickness of eliminated part is greater in the center of the cornea than in the periphery, as the eliminated part being convex lens-shaped. To eliminate the cornea as mentioned above, a regular diaphragm with variable circular aperture is utilized to change the ablation area by the laser beam on the cornea, thereby correction of myopia may be relatively easily performed.
In the case of correcting hypermetropia, contrarily, as shown in FIG. 2, the surface of a cornea E.sub.c has to be eliminated so that the thickness of eliminated part is greater in the periphery of the cornea than in the center, as the eliminated part being concave lens-shaped. Accordingly, to eliminate the surface of the cornea, an aperture diaphragm must cut laser beam to ablate the cornea off in its center part and also vary the radius size of aperture, but it is difficult for a regular diaphragm with circular aperture.
To control diaphragm utilized for correction of hypermetropia, several methods have been proposed. For instance, Japanese Patent Publication No. HEI 4(1992)-33220 based on the United Kingdom Patent Application No. GB 8606821 (Applicant: Summit Technology, Inc.,) which corresponds to U.S. Pat. No. 4,994,058, declares a method of ablating the surface of a cornea as concave lens shape with a special mask, as shown in FIG. 3. The mask provides a predefined profile of resistance to laser beam, the profile being created by varying the thickness or the composition of the mask. When laser beam is passed through the mask to irradiate the cornea, part of the laser beam is selectively absorbed and part is transmitted to the cornea in accordance with the mask profile to selectively ablate the surface. The mask for correction of hypermetropia is consisted of a center part which more absorbs and less transmits laser beam and a peripheral part which more transmits and less absorbs laser beam, so that the surface of the cornea is eliminated as concave lens shape.
Japanese Laid-Open Patent Publication No. SHO 64(1989)-86968 based on French Patent Application No. FR 8708963 corresponding to European Patent Publication No. 0 296 982 (applicant; International Business Machines Corporation) proposes another method of eliminating the surface of the cornea by laser beam, in which laser beam is irradiated to the cornea through a diaphragm with lobe-shaped aperture, while the aperture of the diaphragm being displaced in rotation or in translation over the area of the cornea to be corrected. The lobe-shaped aperture of the diaphragm is formed in a predefined shape. A plurality of the images of the lobe of the laser beam passed through the aperture of the diaphragm are intermittently formed on the cornea to eliminate the surface of a cornea by thickness necessary to correct ametropia, accordingly, the curvature of the cornea is changed according to eliminated part of the cornea. The aperture of the diaphragm utilized for correction of hypermetropia is formed in a lobe shape having large width at a part corresponding to the peripheral part of the cornea, so that the periphery of the cornea may be ablated more greatly than the center.
Another method of correcting ametropia using a diaphragm with lobe-shaped aperture, which is similar to the above Japanese Laid-Open Patent Publication No. SHO 64(1989)-86968, is described also in Japanese Laid-Open Patent Publication No. HEI 2(1990)-84955 based on U.S.S.R. Patent Application No. SU 4457772 which corresponds to U.S. Pat. No. 4,953,969.
However, in a case of correction of hypermetropia, there are following problems in the above prior methods of correcting ametropia.
In the first method using a special mask, the shape of the mask is variously required according to curvature of cornea and correction degree of refractive power of the eye to be treated, accordingly, a number of masks must be ready in various shapes for every curvature and correcting refractive power of the eye before treatment. The eliminated thickness of the cornea depending on the shape of the mask, precision of the shape of the mask is important factor in a case of correction of ametropia, and manufacture of the mask is difficult accordingly.
In the second method of displacing images of the lobe of the aperture, the shape of the aperture required is various according to curvature and correcting refractive power of the cornea to be treated, similarly as in the above method, a number of diaphragms have to be provided according to the eye to be treated.